Unable to use 60 Hz mode on HP S270n

[Maqqe]

Dear HP support community,

I have a HP S270n hooked to my MacBook Pro at work. Initially I was able to use the 60 Hz mode on the display just fine, but now I am locked out of it and can only use the display at 30 Hz. I've tried both direct USB-C connection from the display to my laptop and also tried using the HDMI 1 port on the display, connected to the laptop thru a HDMI-to-USB-C dongle using the HDMI cable that came with the display.. In both cases the current mode in the display's settings is set to 3840x2160 30 Hz, while the optimal mode is 3840x2160 60 Hz.

I do not know what caused the 60 Hz option to become unusable, it just happened a while back and I am unable to get back to the 60 Hz mode. Using the display at 30 Hz is very annoying indeed and I'd like to get back to using the 60 Hz mode if possible.

Thanks in advance for any advice you may have.

[decisions]

Same thing happened to me. Using a MBP 2018 on Big Sur. I had been using the 4k 60Hz setting just fine for many months and really enjoyed this monitor. 

Then one day all of a sudden it is stuck at 30Hz. There is no option to change the refresh rate in the display settings. If you download a popular app called EasyRes, it shows all the available settings for your display. For some reason the 4k 60Hz setting has disappeared from here and now the highest 60Hz seeing is 1080p. In the monitor system settings, it still lists the Optimal Mode correct at 3840x2160 60Hz like Maqqe said. 

This is my main workstation everyday and now out of nowhere my 4K monitor has been downgraded to a 1080p one. Please help. Thanks.